ADM1024
Suitable fan drive circuits are given in Figures 16a to 16f. When
using any of these circuits, the following points should be noted:
4. The op amp may be powered from the 12 V rail alone or from
12 V. If it is powered from 12 V then the input common-
mode range should include ground to accommodate the
minimum output voltage of the DAC, and the output voltage
should swing below 0.6 V to ensure that the transistor can
be turned fully off.
1. All of these circuits will provide an output range from zero
to almost 12 V, apart from Figure 17a which loses the base-
emitter voltage drop of Q1 due to the emitter-follower
configuration.
5. If the op amp is powered from –12 V, precautions such as a
clamp diode to ground may be needed to prevent the base-
emitter junction of the output transistor being reverse-biased
in the unlikely event that the output of the op amp should
swing negative for any reason.
2. To amplify the 2.5 V range of the analog output up to 12 V,
the gain of these circuits needs to be around 4.8.
3. Care must be taken when choosing the op amp to ensure
that its input common-mode range and output voltage swing
are suitable.
12V
12V
R1
100k⍀
R2
100k⍀
+
1/4
LM324
AOUT
Q3
IRF9620
Q1
2N2219A
R2
36k⍀
R3
3.9k⍀
R1
10k⍀
AOUT
Q1/Q2
MBT3904
DUAL
R4
1k⍀
Figure 16d. Discrete Fan Drive Circuit with P-Channel
MOSFET, Single Supply
Figure 16a. Fan Drive Circuit with Op Amp and Emitter—
Follower
12V
12V
R2
100k⍀
R4
1k⍀
+
1/4
LM324
AOUT
Q1
BD136
2SA968
Q3
IRF9620
R3
1k⍀
R3
39k⍀
Q1/Q2
MBT3904
DUAL
R2
39k⍀
AOUT
R4
10k⍀
R1
10k⍀
R1
4.7k⍀
–12V
Figure 16b. Fan Drive Circuit with Op Amp and PNP Transistor
Figure 16e. Discrete Fan Drive Circuit with P-Channel
MOSFET, Dual Supply
12V
12V
R1
100k⍀
R2
100k⍀
R3
100k⍀
+
1/4
LM324
AOUT
Q4
Q1
IRF9620
BD132
TIP32A
Q3
BC556
2N3906
R2
39k⍀
R3
3.9k⍀
R1
10k⍀
AOUT
Q1/Q2
R5
100⍀
R4
1k⍀
MBT3904
DUAL
Figure 16f. Discrete Fan Drive Circuit with Bipolar Out-
put Dual Supply
Figure 16c. Fan Driver Circuit with Op Amp and P-Channel
MOSFET
REV. 0
–14–
ADI [ ADI ]
